Low battery behavior - nicad vs lithium.

Is the behavior of cordless tools as their batteries run down different for nicad vs lithium batteries?

I notice that with a nicad battery in my cordless weed trimmer, the tool gradually slows down as the battery gets low. The same is true of the nicad batteries for my other cordless tools.

With lithium batteries in the weed trimmer, the tool just stops when the battery gets low. When I press the switch again it may run for a short burst and then stop again. When the lithium batteries for the cordless lawn mower get low, the machine just stops. (I haven't used lithum batteries in other cordless tools enough to notice their behavior.)

Yep, I notice the same thing. Tools with Li-ion batteries generally just stop. I may see a bit of slow down just before it dies but not much time.

Mike

Yes I have both and the Li-ion just dies no warning. the bad thing is I will be using it and climb a ladder to do something and it dies just as I start.

My Li-ion batteries hold their charge for a long time where my nicad batteries do not. Ni-cads have a memory as well where the Li-ion batteries don't. I have several nicad Roybi batteries that I have not been able to get the use out of them that I should have. I won't buy any more Roybi products for that reason. My old Makita nicads lasted forever and I may go back to that brand when I replace my Roybi tools. The bad thing is I have at least 6 Roybi tools that have seen little use except for the drill.
I have one of the Makita small drill/screwdriver and it is awesome. The battery does stop without warning but the tool came with a spare battery so I pull the dead battery out,stick it in the rapid charger, load the spare battery and before the spare is used up the other is fully charged.

Most newer tools using li ion have circuitry in the tool to protect the battery from going to far into discharge. The reason the tool stops is the protection has kicked in and cut off power to the tool.

li ion batteries can catch fire or completely fail if under or over charged.

That's completely normal for LI packs. The circuitry monitors voltage and temp, shutting down if either is outside a predetermined range. That circuitry, as well as smarter chargers, are essential with the LI chemistry due to the high fire risk if the operating envelope is exceeded. So although it's primarily a safety thing, it also prevents shortened service life due to over charge/discharge damage so common with NiCad / NiMh chemistries.

The voltage of a NiCd pack is constantly dropping as you use it so you notice the motor getting slower and slower and you learn to anticipate just how much longer you have left. Li packs see a slight voltage drop then they hold pretty much the same voltage till they are depleted at which point the voltage drops like a rock which causes your tool to suddenly stop working. My Li powered drill has lights on the side to tell me how full the pack is and they work pretty well so I swap packs before I reach that cliff where the tool suddenly stops. It's really no worse than using a gas powered tool.

Running NiCd batteries until they're totally dead severely shortens their useful life. You're supposed to put them on the charger once you notice the performance starting to drop off. And, yes, their self-discharge rate is fairly high.

Same experience here; lithiums just stop but, hold a charge much longer. I also agree that running your other batteries at all once they start to show signs of a weakening charge only kills them faster.

I've always had the best discharging the Ni Cads all the way. I used drills the most used tool when I was installing drug store fixtures. I always carried 6 drills on my cart with a different bit in each one. Had 12 drills that would go with me to the job 24 batteries some lasted 12 years most at least 8 years doing this and that was day in and day out.

The voltage of all rechargeable batteries starts out at a peak value when freshly charged, gradually decreases as you use the battery, and then dives rapidly when the battery is fully discharged. Nickel-based batteries' voltage drops more during the "gradual decrease" phase than Li-ion, so you will notice slightly more decrease in oomph during a run.

Because it is impractical to perfectly match the cells in a battery, one of them always discharges fully before the others. If you continue to use the battery after this happens, you force current through the flat cell ("cell reversal"), which causes it to become hot and can permanently damage it. However, with Li-ion there is an additional peril: Li-ion chemistry is prone to thermal runaway. That is, heat causes chemical reactions in the cell to increase, which creates more heat. This positive feedback can cause the cell to catch fire, explode, or vent red-hot gasses (think about those grounded 787s). As a result, Li-ion batteries always have safety sensors that (are supposed to) shut them down if they become hot and usually that shut them down when they are nearly fully discharged. This means that a Li-ion battery will suddenly stop altogether when discharged. Nickel-based cells also get hot when reversed, but they don't experience thermal runaway and therefore don't need the safety circuitry. However, the reversal and heat can still permanently damage the battery. With Ni batteries, you should stop and recharge immediately when you experience the sudden drop in performance at full discharge.